Most common mistake in ERP implementation

Consider a big organization with fully developed and established legacy systems. Organization has been running its businesses for so many years with fully developed and matured legacy systems. Now with scaling up of operations and a need to achieve better harmonization amongst various departments of organization, management has deemed it fit to go out for an ERP implementation.
Management is under pressure and wants to go ahead with ERP implementation at a fast pace. The implementation team, the trainers and the persons from the organization know that given time line is not feasible. Team has done an excellent exercise and prepared a fine blueprint and the roadmap.